1. Field of the Invention
The present invention relates generally to the field of systems for fracturing the strata surrounding a well. More specifically, the present invention discloses a system for fracturing oil and gas wells using supplemental longer-burning propellants.
2. Statement of the Problem
Propellant fracturing has been used in the oil and gas for over 25 years with varying degrees of success. The burn time of most propellants have generally been very short (i.e., on the order of a few milliseconds to as much as 100 milliseconds). Such short burn times limit fracture propagation in the strata surrounding the well and increase the likelihood of damage to the well and the well casing. It is also more difficult to accurately model the combustion and fracturing processes in such a short time frame.
3. Prior Art
The prior art in the field of the present invention includes the following:
InventorPatent No.Issue DatePassamaneck5,295,545Mar. 22, 1994Trost4,798,244Jan. 17, 1989Hill et al.4,718,493Jan. 12, 1988Hill et al.4,633,951Jan. 6, 1987Hill et al.4,683,943Aug. 4, 1987Hane et al.4,329,925May 18, 1982Godfrey et al.4,039,030Aug. 2, 1977
Passamaneck discloses a method of fracturing wells using propellants which burn radially inward in a predictable manner. A computer program is used to model the burn rate of the propellant to determine a suitable quantity and configuration of the propellant for creating multiple fractures in the surrounding formation.
Trost discloses a tool for radially fracturing the rock formation surrounding a well bore using a perforated cylindrical canister housing a stack of propellant modules.
The patents to Hill et al. disclose a fracturing system in which a cased well is initially filled with a compressible hydraulic fracturing fluid containing a mixture of liquid, compressed gas, and propant material. The fracturing fluid is precompressed in the well. The well casing is then perforated, which releases the precompressed fracturing fluid to fracture in the surrounding formation.
Hane et al. disclose an apparatus for explosive fracturing in which opposed end charges are detonated to enhance the explosive capability of a central explosive charge.
Godfrey et al. disclose a system for stimulating production in a well that is first filled with a fracturing fluid. A high-explosive charge is then suspended in the well adjacent to the pay zone. A propellant is suspended in the well above the high-explosive charge. The propellant is ignited first, followed by detonation of the high explosive. The purpose of the propellant is to maintain pressure caused by the high explosive over a longer period of time, thereby extending the fractures caused by the high explosive.
4. Solution to the Problem
In contrast to the prior art, the present invention employs a combination of a new ignition method and a propellant engineered to have longer burn times to produce burn times ranging from 400 milliseconds to several seconds. The present invention uses a propellant system that employees longer burns in combination with additional propellant placed above or below the primary propellant grain. The primary propellant has a burn time tailored so that the pressure remains above the maximum fracture extension pressure but not so large as to damage the well casing. The ignition of the primary propellant produces a pressure rise time that falls in the multiple fracture regime of the formation being fractured. The burn time for the primary propellant is from 400 milliseconds to approximately 1 second. However, the time that the propellant creates fractures parallel to the minimum stress plane is only 40 to 45% of the times mentioned above. The addition of supplemental propellant grains to sustain gas production after the primary propellant burn is complete allows the fracturing process to continue for durations of as long as 20 seconds. This approach allows fractures to continue their extension into the formation for times that are much longer than for a single propellant grain, thus increasing the effective fracture lengths and the corresponding effective well bore diameters.